Please refer to FIG. 1 which is a circuit diagram of a conventional line-interactive uninterruptible power supply. In FIG. 1, the conventional line-interactive uninterruptible power supply contains an AC input voltage AC, a switch containing two diodes D1 and D2, a single-phase AC/AC converter 11, an AC filter 12 containing a filter inductor Lo and a filter capacitor Co, and a load R.
The single-phase AC/AC converter 11 contains an AC inductor Li, a bus capacitor Cs and three arms. The three arms are respectively a boost arm comprising a switch 51 and a switch S2, a common arm comprising a switch S3 and a switch S4 and a buck arm comprising a switch S5 and S6.
An AC input voltage AC directly provides energy to the load R by the line-interactive uninterruptible power supply 1 of FIG. 1 when the AC input voltage AC (mains electricity) is operated normally. The boost arm and the common arm execute a rectifying function. When the AC input voltage functions abnormally, a storing battery (not shown) provides energy to the load R. At this time, the common arm and the buck arm execute an inverting function.
In fact, two switches S3 and S4 of the common arm are controlled by network frequency. The switching frequency is low-frequency switching. Therefore, the boost arm is used as a rectifying device and the buck arm is used as an inverting device.
In the prior arts, in order to implement regulation of the AC input voltage, several controlling methods could be used. The difference between the controlling methods was to generate bus voltages having different waveforms. The following explanations show two kinds of common methods of controlling the bus voltages having different waveforms.
Please refer to FIGS. 2(a) and 2(b) which were waveform diagrams of bus voltages of an uninterruptible power supply of FIG. 1 for different controlling methods. The FIGS. 2(a) and 2(b) showed respectively waveforms of different bus voltages. The bus voltage of FIG. 2(a) had a DC voltage waveform while the bus voltage of FIG. 2(b) had a full-wave rectifying voltage waveform. Certainly, the different bus voltages generated under different controlling methods had advantages and drawbacks.
In order to obtain the DC bus voltage shown in FIG. 2(a), the buck arm and the boost arm must be operated under a high frequency pulse-width-modulation mode and the common arm must be operated at low frequency switch state. When the AC input voltage AC was a positive half-wave, the switch S4 turned off. When the AC input voltage AC was a negative half-wave, the switch S3 turned off. An unit input power factor was obtained by using a method of calculating an input current and an input voltage so as to obtain a duty cycle of the buck arm of a single-phase AC/AC converter 11. Therefore, a DC voltage was controlled by controlling an increasing amount of an input reference current.
A main advantage of this controlling method was that the input current could be controlled to be a sine-wave so as to obtain an unit input power factor. An output voltage could be accurately regulated. A no-load current would decrease. On the contrary, a main drawback was that a switch loss was large and a an efficiency was poor (especially under full-load).
In order to obtain a full-wave bus voltage shown in FIG. 2(b), when the AC input voltage AC was abnormal, a converter 11 was operated at a boost state, the switches S1 and S2 of the boost arm were operated under high frequency pulse-width-modulation mode. When the converter 11 was operated at a buck state, only the switches S5 and S6 of the buck arm were operated high frequency pulse-width-modulation mode. Besides, a capacitance of a bus capacitor Cs was small, then a bus voltage having full-wave rectifying voltage waveform shown in FIG. 2(b) was obtained.
A main advantage of this controlling method was that the volume could be decreased, reliability of the circuit was enhanced and usage life of the circuit was lengthened by using a high frequency film capacitor as the bus capacitor Cs. Besides, a loss of the switches was small when a design of the controlling unit was simplified.
A main drawback of this controlling method was a poor dynamic performance, a large reactive current of input, and a large ripple current of the bus capacitor Cs. Because a current flowing through the load R during light-load and no-load periods was small, in order to maintain the full-wave rectifying voltage waveform of the bus capacitor Cs, an inefficient charging and discharging on the bus capacitor Cs was carried out. But, a large loss of the converter 11 was generated during light-load or no-load period.